You do go on the HM during KTTK the way you get into the attraction is via the "Entrance Quarters". BTW, if you get Michael or Matthew as your tour guide you are in for a treat. They are both very good. Out of the 2 Matthew is my fav.
Is Michael the tour guide with the slight stutter? (Sorry, it's the only thing I can remember him by) If so, I'm glad to hear he's still giving tours. We had him 8 or 9 years ago for our KTTK tour, and he was GREAT!
As for the hallways, as mentioned, there are two. Both doors facing the exit walkway are labeled "Servants Quarters, Do Not Enter" or something to that effect.
The longer one, closer to the bottom of the diagram posted is themed as a servants' hallway with the call bells from various rooms in the Mansion and keys on the wall. It attaches to the foyer with the aging portrait. (It's opposite the door you normally enter as a Guest from the queue) You can only "legally" access this hallway accompanied by a CM, because the cast breakroom is off of it, as well as a door to stairs up to the upper level where the cast restrooms are. This is used when a CM wants to put Guests into the sequence of the show starting with the foyer so that they get the full stretching room show. (Or perhaps by a family with a REALLY REALLY scared child who wants OUT of the Mansion NOW....)
The other, shorter hallway is actually accessible to Guests on their own, though really only for one-way traffic (start of ride to end of ride). The door to this hallway from the boarding area has a rather large sign on it that says that if you are too frightened to ride, it's your last chance to exit. So because it's kind of a self-serve exit, it makes me assume that there are no off-limits areas accessible to a Guest walking through this hallway (or if there are, it's a locked door). It's simple a way of getting from load to unload. But, CMs apparently also use this hallway for bringing handicapped Guests and VIPs who are OK skipping the stretching room to the boarding area of the attraction.
I just don't know what the theming of this second hallway looks like, as I've never been through that one. Just the first one.
So theoretically, if you wanted to see this shorter hallway, you're OK to use it to exit the Mansion after the stretching room. You just have to "sacrifice" riding the doombuggy portion of the attraction.
EDIT: I believe the video referred to by the OP is this:
http://www.youtube.com/watch?v=zc570IA5_Lk
This video is of the longer hallway connecting to the foyer. Note that you can hear the Ghost Host's stretching room entrance speech in the background. And the door that the cameraman walks up to at the end of the clip is the door to the exit walkway.
-Rob